How Do I Get Rid of St. Augustinegrass in a Flowerbed?

Unwanted grass competes with your flowers for water and nutrients.

St. Augustine (Stenotaphrum secundatum ) is a warm-season, perennial turfgrass used commonly for lawns throughout U.S. Department of Agriculture plant hardiness zones 8 through 10. Known for its attractive, lush appearance, the grass spreads rapidly through creeping stolons to cover lawn areas. However, those same runners that make St. Augustine varieties attractive lawn covers often cause headaches when they grow into your flowerbeds. Chemical and organic solutions exist to help you kill the errant grass without harming your ornamental plants.

Preplanting Site Prep

Treating your flower bed with a glyphosate-based herbicide about two weeks before you plant ornamentals can help control the St. Augustine grass actively growing in the bed. One glyphosate-based product recommends using 5 tablespoons of product for every gallon of water, but follow label instructions, since directions do vary. Use a handheld garden sprayer to cover the undesirable grass thoroughly with the solution. Spraying the grass on a sunny, warm day helps accelerate the weed-killing process. Reapply the herbicide if your area experiences rainfall within 24 hours of treatment. Tilling the treated area immediately after spraying helps work the chemicals down into the soil so they can kill the grass roots quickly.

Treating Around Ornamentals

Using a glyphosate-based herbicide also can help control St. Augustine grass encroaching into an already planted flowerbed, but you must use the product carefully around your desirable plants. Following the manufacturer's instructions, mix about 5 tablespoons of product into 1 gallon of water. Dip a small, foam-tipped paint brush into the solution and apply the herbicide carefully directly onto all visible grass blades. The glyphosate will move throughout the plant to kill the roots without harming your ornamentals. Once the St. Augustine dies, pull the grassy remains out of your flowerbed and repeat the process for new growth, if necessary.

Using Glyphosate

Glyphosate can cause eye or skin irritation, so reduce your risk of exposure by wearing waterproof gloves, protective eyewear, long sleeves, pants and closed-toe shoes. Because this systemic herbicide works best if the grass is growing actively, apply the solution as soon as you notice the errant runners. Glyphosate is non-selective, which means it kills most plants. It works by preventing the plants from producing specific proteins necessary for growth. Because the herbicide won't discriminate between weeds and ornamentals, promptly rinse off any product that splashes or drifts onto desirable plants.

Non-Chemical Control Measures

Hand pulling St. Augustine runners effectively removes any grass growing in your flower bed. Mulch can kill any errant runners by depriving the grass of the sunlight it needs to grow. North Carolina State University recommends using a fine-textured mulching material in flowerbeds, so consider applying pine bark or needles, decomposed leaves, straw, compost or hardwood bark around your plants. Make sure the mulch layer lies 1 to 3 inches thick to block the light effectively, but keep the material a few inches away from the plant bases to prevent attracting pests or disease pathogens.